The earliest rolling cutter earth-boring bits had teeth machined integrally from steel, conically shaped, earth disintegrating cutters. These bits, commonly known as “steel-tooth” or “mill-tooth” bits, are typically used for penetrating relatively soft geological formations of the earth. The strength and fracture-toughness of steel teeth permits the effective use of relatively long teeth, which enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel-tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks.
Consequently, it has been common in the art since at least the early 1930s to provide a layer of wear resistant metallurgical material called “hardfacing” over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macrocrystalline tungsten carbide dispersed in a steel, cobalt or nickel alloy binder or matrix. Such hardfacing materials are applied by heating with a torch a tube of the particles that welds to the surface to be hardfaced a homogeneous dispersion of hard particles in the matrix. After hardfacing, the cone is preferably heat treated, which typically includes carburizing and quenching from a high temperature to harden the cone. The particles are much harder than the matrix but more brittle. After hardening, the matrix has a hardness preferably in the range from 53 to 68 Rockwell C (RC). The mixture of hard particles with a softer but tougher steel matrix is a synergistic combination that produces a good hardfacing. There have been a variety of different hardfacing materials and patterns, including special tooth configurations, to improve wear resistance or provide self sharpening.
FIG. 1 shows a prior art mill-tooth bit 11. Earth-boring bit 11 includes a bit body 13 having threads 15 at its upper extent for connecting bit 11 into a drill string (not shown). Each leg of bit 11 may be provided with a lubricant compensator 17. At least one nozzle 19 may be provided in bit body 13 for directing pressurized drilling fluid from within the drill string and bit 11 against the bottom of the bore hole.
Cones 21, 23, generally three (one of which is obscured from view in FIG. 1), are rotatably secured to respective legs of bit body 13. A plurality of inner row teeth 25 and outer row teeth 27 are arranged in generally circumferential rows on cones 21, 23, being integrally formed on the cones, usually by machining. Outer or heel row teeth 27 are located at the outer edges of each cone 21, 23 adjacent gage surfaces 29. Each bit leg has a shirttail portion 31 on its outer side adjacent gage surface 29 of cones 21, 23. Typically, hardfacing will be applied to inner row teeth 25, heel row teeth 27, gage surface 29 and also to shirttail 31.
FIGS. 2 and 3 illustrate a tooth 28 that typically would be in a heel row in place of heel row 27 in cone 21 of FIG. 1. Tooth 28 is formed with a milling cutter which forms a root 43, inclined flanks 33, 35 and an elongated crest 37. An outer or gage end 39 is located at the outer side adjacent gage surface 29 (FIG. 1), and an inner end 41 is located opposite outer end 39. Hardfacing 45 is applied to the flanks 33, 35, and crest 37. Tooth 28 has a centerline 49 (FIG. 3) which is substantially symmetrical and bisects tooth 28. Centerline 49 extends through the axis of rotation of cone 21.